PROJECT SUMMARY: Many infectious diseases exhibit differential severity between the sexes yet the mechanisms driving such innate biases are incompletely understood. Staphylococcus aureus (SA), including methicillin-resistant (MRSA), is the most common cause of skin and soft tissue infection (SSTI) in the US, and males have a greater than 2-fold higher incidence of infection versus females. Neutrophils are essential for clearance of SA skin infections, and we found that, consistent with murine infection outcomes, neutrophils from female mice are better able to clear SA ex vivo compared to neutrophils from male mice. Major contributors to neutrophil killing of SA include oxidative burst, antimicrobial factors in neutrophil granules, and neutrophil extracellular traps (NET). Therefore, in this exploratory proposal, we aim to align the efficacy of neutrophil phagocytosis and killing of SA ex vivo with measurements of reactive oxygen species (ROS) and nitric oxide (NO) production, NET formation, and changes in gene and protein expression and activation, to define the mechanism(s) of sex-specific phagocyte efficacy against SA. Furthermore, we have identified sex-specific modulators which enhance phagocyte efficacy and will determine the mechanisms by which these modulators augment innate defense. Upon the successful completion of these aims, we will have gained significant insight into the mechanisms of sex-specific phagocyte efficacy against SA, as well as mechanisms and/or targets for augmenting host innate defense.